Mobile communications systems, such as those used for cellular telephone communication, divide the available frequency spectrum into a multiplicity of individual signalling channels or frequency bands. Particular channels are allocated to individual users as they access the system. Each user's communications are routed by the system through the channel allocated to that user. Signals broadcast by the system must be carefully regulated so that they remain within the channels allocated to the various users. "Out-of-band" signals can spill over from one channel to another, causing unacceptable interference with communications in the other channels.
To date, mobile communications systems have generally employed frequency modulation ("FM") techniques which do not require variation of the amplitude of the transmitted signal. Thus, the frequency of the transmitted signal carrier changes, but the signal power level remains constant. Such systems have sufficed for voice communications. However, there is an increasing desire to expand the capabilities of mobile communications systems to encompass data as well as voice communications. Commercially worthwhile data transfer rates require the use of modulation techniques which are more spectrally efficient than the FM techniques used for voice communication. This necessitates the use of amplitude modulation techniques which in turn require linearized modulation.
The electronic amplifiers employed in any communications system inherently distort signals as they amplify the signals. FM techniques do not suffer from such distortion because of their constant amplitude. Amplitude modulation, however, causes the distortion to become dependent on the input signal, so that the amplifier output signal is no longer simply an amplified replica of the input signal. Although an amplifier's input signal may be confined within a particular channel or frequency band, the distorted, amplified output signal typically includes out-of-band frequency components which would overlap one or more channels adjacent to the channel within which the input signal was confined, thereby interfering with communications in the overlapped channel(s).
Some degree of channel signal overlap due to unregulated amplifier distortion is acceptable in some cases. However, mobile communications systems place very stringent restrictions on out-of-band signal emissions in order to minimize channel-to-channel interference.
To reduce out-of-band signal emissions to an acceptable minimum the amplifier input signal is conventionally "predistorted" before it is fed into the amplifier. Before the signal is amplified, an estimate is made of the manner in which the amplifier will inherently distort the particular input signal by amplifying that signal. The signal to be amplified is then "predistorted" by applying to it a transformation which is estimated to be complementary to the distorting transformation which the amplifier itself will apply as it amplifies the signal. In theory, the effect of the predistorting transformation is precisely cancelled out by the amplifier's distorting transformation, to yield an undistorted, amplified replica of the input signal. Such amplifiers are said to be "linearized" in the sense that the output signal is proportional to the input signal, thereby eliminating the generation of out-of-band components.
Unfortunately, amplifier distortion varies in a complex, non-linear manner as a function of a wide range of variables, including the amplifier's age, temperature, power supply fluctuations and the input signal itself. Accordingly, it is not possible to define a single predistortion transformation which will cancel out any and all distorting transformations applied by the amplifier.
One prior art approach to the problem (exemplified by U.S. Pat. No. 4,462,001 issued July 24, 1984 for an invention of Henri Girard entitled "Baseband Linearizer for Wideband, High Power, Nonlinear Amplifiers") has been to construct a look up table containing a multiplicity of entries which define predistortion transformation parameters appropriate for use with a corresponding multiplicity of different input signals. That is, the effects of the amplifier's distortion on a range of input signals are pre-measured, the complementary predistorting transformations corresponding to each input signal are calculated, and parameters defining the calculated complementary transformations are stored in the table. In operation, the fluctuating power level of the signal to be amplified is continuously measured. The power measurement is then applied to the electronic embodiment of the table, from which the corresponding predistortion parameters are derived, so that the input signal sample may be predistorted before it is fed to the amplifier. However, Girard's approach accounts only for variation of the input signal, not for variation of the amplifier's other distorting characteristics. Because the amplifier's other distorting characteristics in fact vary it is necessary to continuously "adapt" the lookup table parameters by changing them in response to changes in the amplifier's other distorting characteristics.
Moreover, Girard's approach is based on separate tables containing amplitude and phase correction factors. This "polar coordinate" representation follows naturally from the common practice of representing amplifier distortion in terms of AM/AM and AM/PM characteristics. So far as the inventor is aware, all predistortion techniques prior to Girard's also attempted separate amplitude and phase correction.
Another prior art approach is exemplified by U.S. Pat. No. 4,700,151 issued Oct. 13, 1987 for an invention of Yoshinori Nagata entitled "Modulation System Capable of Improving a Transmission System". Nagata uses the real and imaginary quadrature components of the input signal sample to index into a lookup table containing predistortion transformation parameters. The real and imaginary components are each typically defined by at least 10 bits of information. Thus, Nagata employs a 20 bit index, which requires a lookup table containing 2.sup.20 entries (i.e. over 1 million entries). The lookup table entries are adaptively changed in response to variations in the amplifier's distorting characteristics. However, if the signalling channel is changed (a common occurrence in mobile communications systems) then every entry in Nagata's lookup table must be iteratively recalculated. This process can take 10 seconds or longer, which is unacceptable.
The present invention overcomes the disadvantages of the prior art. By storing table entries in rectangular coordinate format, it enables the subsequent predistortion operation to be performed more simply than Girard's polar coordinate approach. Further, it adapts to amplifier and oscillator changes, whereas Girard's predistorter does not. In comparison with Nagata's method, only a comparatively small lookup table is required. Signal phase rotators (required to stabilize Nagata's circuitry) are not required. Moreover, the lookup table entries are adaptively changed, within about 4 milliseconds ("msec.") in response to changes in the amplifier's distorting characteristics.